1. Field
The present invention relates generally to methods, devices and systems for reselecting and then handing over a mobile communications device from a first cell to a second cell in a cellular wireless communications system. More particularly, although not exclusively, aspects and embodiments of the invention relate to criteria for selecting a second cell while a mobile station is ‘camped’ on, or otherwise interacting with and/or controlled by, a first cell. Particular aspects and embodiments of the present invention are well suited for use in a cellular wireless communications system which supports packet switched communications, for example according to the General Packet Radio Service (GPRS) standard, but are not limited to such an application.
2. Background
It is well known that cellular wireless communication systems generally comprise a number (often large) of radio transceivers, or base stations, that define service areas or cells. The schematic diagram in FIG. 1 of the accompanying drawings, illustrates a system 100 comprising four base stations 120 defining respective cells 110. The cells typically overlap in order to ensure continuous coverage of service in the service areas. This is desirable for many reasons, not least because cellular systems are designed specifically to accommodate users as they move around within the system. In principle, mobile communications devices 130 interact with various base stations as the devices move through the respective cells 110 of the system 100.
One of the goals of a cellular wireless communication system is to enable a mobile communications device, which will be referred to herein for convenience as a “mobile station”, to remain connected to the system even when the user is moving through the system from one cell to another. Traditionally, the mobile station has been a so-called “mobile phone” or “cellular phone,” although, with advances in technology, a mobile station may be any one or more of a wide range of devices from solely voice devices to solely data devices. A mobile station may be anything from a traditional radio pager or mobile phone, though faxes, personal data assistants (PDAs), and music players, to computers, or any combination of these. This list is, of course, far from exhaustive. Indeed, although the term “mobile station” is used herein, the term is also intended to encompass devices that may not be user-operated or even user-operable, for example the device could be a wireless ‘data card’ or the like, which is within another kind of apparatus.
Early cellular systems were circuit switched systems. That it to say, for each call the system created a circuit that reserves a channel for the user for the duration of the call. This is an inefficient use of resources, especially for bursty data. As technology has advanced, newer cellular systems have moved away from circuit switching to packet switching in which bursts of data are sent only when needed. Consequently, cellular systems have become more suitable for the transmission of data, which tends to be transmitted in bursts rather than a continuous stream.
As already mentioned each cell in a cellular system is defined and served by a base station. As a mobile station is moved from the service area defined by one cell into that defined by another, the system and the mobile station must break the connection with one base station and establish a connection with another base station whilst minimizing the connection loss between the mobile station and the system. This operation is sometimes known as a cell reselection, a handoff or a handover. For simplicity of description only herein, the term “reselection” will be used as a generic term to describe the operations involved with a mobile station or equivalent moving from operating with one base station to operating with another base station; and the reader should import an alternative term, such as “handover”, “handoff” or the like, if the context so dictates. The term “camped on” is commonly used, and will be used hereinafter, to describe the base station with which, and respective cell in which, a mobile station is operating. That is, a cell reselection involves a mobile station moving from being camped on one cell to being camped on another cell.
Typically, a cell reselection can be initiated either by the mobile station or by the cellular system. How reselection is initiated can depend on factors such as the kind of cellular system, its mode of operation and on the capabilities of a mobile station. In any event, reselection is typically initiated either as a result of a service degradation, which tends to lead to increased power consumption requirements, or there being an opportunity to improve the service, which would lead to reduced power consumption requirements. Especially since many mobile stations operate from battery power, an opportunity to reduce power consumption, thereby improving power efficiency, is usually advantageous. Service degradation can result from factors such as increasing distance between a mobile station and a base station or natural or man-made obstructions such as hills or buildings respectively.
One known kind of reselection operation requires a mobile station to monitor the signaling level and suitability of cells that neighbor the cell on which the mobile station is camped, which will be referred to hereinafter as the “serving cell”, and compare the monitored service levels with the signaling level and suitability of the serving cell. Then, if the signaling level and suitability of a neighboring cell is deemed by the mobile station to be better than that of the serving cell, for at least a predefined period of time (say, five seconds), the mobile station initiates a reselection to the respective neighboring cell, which becomes the new serving cell. Such an operation is described in an ETSI Technical Specification document 145 008 v4.16.0, Digital Cellular telecommunications system (phase 2+); Radio subsystem link control (3GPP TS 45.008, version 4.16.0, release 4, section 6.6.2).